1. Field
The present disclosure relates to a light barrier for the detection of an object which interrupts a beam of light, and which has a sensor with an evaluation unit for the evaluation of a reflected light beam.
2. Background
It is known that a sensor is arranged on a side of a conveying device, with said sensor having at least one light transmitter for the transmission of a light beam in the direction of the conveying device and at least one light receiver for the reception of a light beam from the direction of the conveying device as well as an evaluation device for the evaluation of the light beam detected by the light receiver. A retroreflector is arranged on the side of the conveying device disposed opposite the sensor for the reflection of the transmitted light beam. When the transmitted light beam of the sensor is incident on the retroreflector, at least some of the light beam is transmitted back into its starting direction and is detected by the light receiver in the sensor.
In this manner, an optoelectronic sensor device is arranged transversely to the transport direction in the conveying device. If an object is moved into the light beam of the optoelectronic sensor device by the conveying device, the path of the light beam from the light transmitter to the light receiver is interrupted, i.e. the sensor can recognize that the object is located at the point within the conveying device at which the effective light beam of the sensor crosses the conveying device in the non-interrupted case. By this recognition of the beam interruption, the sensor triggers a switch signal which can be used for the carrying out of different actions such as the outputting of a status signal, a mechanical switch position, a weight detection or the like.
A particular demand in the detection of a conveyed object within a conveying device is that the geometrical detection location of the object frequently has to be determined precisely. This demand for a precise detection location has the disadvantage that an optical sensor as a rule has a tolerance zone in this respect which corresponds to the extent of the effective cross-section of the light beam in the transport direction. This can be explained by the fact that, for example, a new sensor having a high-quality retroreflector can only trigger a switch signal in the sensor when the object completely, or nearly completely, covers the cross-section of the light beam, in particular when said sensor and retroreflector are only used at a small distance from one another and do not have any contamination at the corresponding interfaces. In contrast to this, for example, a sensor having a retroreflector attached at a large distance can already trigger a switch signal in the sensor when the object only covers a small partial cross-section of the cross-section of the light beam, in particular when the optical interfaces additionally show high contamination. For this reason, with the known sensors, the cross-section of the light beam of the sensor is frequently kept very small, at least in the extent which is associated with the transport direction.
This is achieved, for example, in that the cross-section of the light beam is kept small both at the light exit and at the light entry at the sensor and in that the course of the cross-section of the light beam over the conveying device and on the retroreflector is strictly limited by means of a corresponding optical transmission and reception system. The geometrical tolerance zone caused by the cross-section of the light beam is thus necessarily considerably restricted. In other words: the finer the optically active cross-section of the light beam, the more precise the geometrical detection location of the object in the conveying device is determined.
This measure of restriction of the cross-section of the light beam has the disadvantage, however, that the adjustment of the sensor with respect to the oppositely disposed retroreflector is complex and necessarily does not only have the consequence of a high time effort for installation, but also a corresponding mechanical effort is also usually necessary in the form of an adjustable holding apparatus. In this respect, this adjustable holding apparatus must also be able to keep the carried out adjustment status stable over a long period, even under rough environmental influences, i.e. in the event of shock load and/or vibration load.